During various refining operations, petroleum feedstocks are often thermally treated by cracking, coking or visbreaking operations at elevated temperatures in order to produce additional liquid product. A side effect from using these high temperatures is, however, the formation of coke. This undesirable not only because it reduces the the yield of liquid products, but also because is has other unfavorable effects upon the process. In visbreaking and thermal cracking a certain amount of coke production has generally been regarded as an unavoidable result but even here the principal objective has been to maximize the yield of high value liquid products and to minimize the coke made, especially the lower value shot coke. There is, therefore, a continuing need to reduce the coking tendencies of petroleum feedstocks.
Although the use of hydrogen donors may be effective in removing contaminates without causing a significant degree of coking as described in U.S. Pat. No. 3,839,187, it is desirable, nevertheless to avoid the use of hydrogen or hydrogen donors and to rely instead upon a simple non-hydrogenative process. More recently, as in U.S. Pat. No. 4,298,455, it has been proposed to minimize the amount of coke or tar and gas formed during a conventional visbreaking process by conducting the process in the presence of a chain transfer agent and a free radical initiator. The free radical initiator reacts with the chain transfer agent to create a free radical which competes with the larger chain hydrocarbons to prevent the formation of tar or coke. Unsaturated hydrocarbons react with the free radical to form saturated hydrocarbons which cannot be polymerized further. This process, of course, requires both the presence of a free-radical initiator and a chain transfer agent.
A primary object of this invention is to provide a process for stabilizing products derived by thermal cracking. Another object of this invention is to achieve such stabilization without the use of chain transfer agents and free radical initiators.